Current loose-fill packing materials, such as expanded polystyrene pellets, are well-known to acquire excessive static electricity, resulting in either being undesirably attracted to or repelled by surrounding objects. Common solutions to relieving the polystyrene pellets of static include either spraying or dipping pre-expanded or expanded polystyrene pellets with antistatic solutions to surface-treat the polystyrene pellets. However, with these methods, the antistatic durability is often inadequate because the coating can peel off, rub off, or be rinsed off the pellets. In addition, uniformity of coating is difficult to achieve, resulting in less than adequate static dissipative properties or higher costs of labor and materials to assure uniformity.
Another prior art method for relieving loose-fill packing materials of their static properties is by mixing an antistatic agent into the pellet material during extrusion. However, this method often results in undesirable nucleation or foaming of the product, reaction with fire retardants or other materials added to the pellet material, and/or poor static dissipative qualities.
Examples of previous attempts to solve the antistatic problem or related problems are described in the following patents:
U.S. Pat. No. 4,808,448 issued Feb. 28, 1989 to H.S. Cox discloses a process of combining an antistatic agent with thermoplastic beads to be pre-expanded in which the antistatic agent is combined in sufficient amounts either: (a) with thermoplastic beads to be expanded prior to injection into a pre-expander and thereafter injecting the combination of the thermoplastic beads and the antistatic agent into the preheated pre-expander for pre-expanding the beads; (b) with the beads in a preheater shortly after injection of the beads into the preheated pre-expander; or (c) in liquid form with dry steam for delivery to a molding cavity of a mold for intermixing with pre-expanded beads. If the antistatic agent to be combined with the bead is in liquid form and is to be mixed with the bead prior to being added to the heated pre-expander, the antistatic agent is mixed with the bead and thereafter the combination is heated to permit the expansion of the beads and atomizing of the liquid antistatic agent filling the space of the environment uniformly coating the expanding bead. Alternatively, if the antistatic agent is in liquid form, the liquid antistatic agent may also be injected into a heated environment separately from the bead wherein it atomizes, filling the environment, uniformly coating the expanding bead when added. Suitable antistatic agents disclosed include quaternized ethoxylated amines, for example, quaternized coconut amine ethoxylate. U.S. Pat. Nos. 4,696,950 and 4,771,081 to H.S. Cox are related to U.S. Pat. No. 4,808,448.
U.S. Pat. No. 4,785,032 issued Nov.15, 1988 to R.E. Touhsaent discloses a water-based antistatic coating composition comprising at least one water-soluble antistatic agent, which may be applied to a polymer film substrate and, upon drying, forms an adherent antistatic coating.
U.S. Pat. No. 4,603,149 issued Jul. 29, 1986 to H.S. Kesling, Jr. et al. discloses that dialkyl bisalkoxylated quaternary ammonium salts, when coated onto the surface of expandable styrene polymer particles, serve as an antistatic and anti-lumping agent for the particles. It is further disclosed that the ammonium salts may be coated onto the styrene polymer by any suitable method, such as dry blending in a mixer or solution coating followed by evaporation of the solvent. The patent teaches that certain quaternary salts may be added to the polymer particles during impregnation of the polymer with a blowing agent.
U.S. Pat. No. 4,393,176 issued Jul. 12, 1983 to W. Lybrand discloses adding chemical components to thermoplastic materials to reduce the electrical resistivity of the material. The chemical component or agent may be an amine when the thermoplastic material is a polystyrene or a modified styrene. The amine is preferably ethoxylated. It is disclosed that the material may be formed into sheets as by extrusion or may be injection molded into different members such as panels. The material may be formed by mixing the ethoxylated amine and the polystyrene and extruding the mixture.
U.S. Pat. No. 4,393,159 issued Jul. 12, 1983 to W. Lybrand discloses polypropylene articles in which the accumulation of static charges is inhibited by adding, e.g., ethoxylated amines to the polypropylene.
U.S. Pat. No. 4,369,227 issued Jan. 18, 1983 to K. Hahn et al. discloses particulate styrene polymers containing blowing agents which are surface-coated with a hydroxycarboxylic acid ester or an ester of a carboxylic acid with an oxyalkylated alcohol. The patent teaches that the esters are present predominantly as a uniformly distributed coating on the surface of the expandable polystyrene particles. The method of application of the coating is not critical; for example, simple tumbling of the finely divided ester with the styrene polymer particles in a commercial mixer may be used. It is also possible to apply the ester from a aqueous dispersion or a solution in an organic solvent, in which case the solvent or water must be removed during application. It is also possible to add the esters to the styrene bead polymerization charge at or towards the end of the suspension polymerization process.
U.S. Pat. No. 4,333,970 issued Jun. 8, 1982 to E.A. Blommers et al. discloses coated styrenic polymer beads which exhibit anti-lumping properties upon pre-expansion produced by forming a suspension of styrenic polymer beads in an aqueous medium and adding thereto, under polymerization conditions, an emulsion which contains a styrenic monomer and a catalyst therefor, and an isoprene or butadiene-styrene macromonomer, the emulsion formed in an aqueous medium containing a polyoxyethylene alkylphenol. The mixture so formed is maintained at an elevated temperature so as to form a polymerized coating about the initial styrenic beads. The beads, so produced, are then impregnated with an expanding agent.
U.S. Pat. No. 4,333,969 issued Jun. 8, 1982 to H.A. Wright et al. discloses a process for forming styrenic polymer beads having a preferred bead diameter from styrenic polymer beads having a smaller diameter which comprises forming a suspension of the small styrenic polymer beads in an aqueous medium with the aid of a finely divided, water-soluble inorganic phosphate suspending agent and a modifier therefor; forming an emulsion of styrenic monomer containing a free radical-producing catalyst in an aqueous medium, using a non-ionic surfactant that is an ethylene oxide condensate of an alkylphenol; and adding the styrenic monomer-catalyst aqueous emulsion to the styrenic polymer bead suspension in polymerizing the styrenic monomer about the styrenic beads.
U.S. Pat. No. 4,314,040 issued Feb. 2, 1982 to A.J. Castro et al. discloses a concentrated antistatic composition adapted for incorporation into various polymers such as olefins prepared by admixing a liquid ethoxylated amine antistatic agent, such as an N, N-bis-(2-hydroxyethyl) alkenyl or mixed alkenyl and alkyl amine, with various polymers such as, for example, polystyrene, heating to form a homogeneous liquid and rapidly cooling the mixture to form a solid antistatic agent. A normally liquid antistatic agent can thus be blended into a polymer such as polystyrene as a dry-solid product to impart antistatic properties to the blended resin. Examples of the amine antistatic agents are diethoxylated tallow (mixed alkenyl and alkyl) amine, and diethoxylated coco amine. U.S. Pat. No. 4,314,040 is a division of U.S. Pat. No. 4,210,556 which is a division of U.S. Pat. No. 4,147,742.
U.S. Pat. No. 4,281,036 issued Jul. 28, 1981 to H. Leithauser et al. discloses fine particulate expandable styrene polymers having coatings applied thereto by: (a) introducing the fine particulate expandable styrene polymers into a closed mixing vessel; (b) reducing the pressure in the mixing vessel below atmospheric; (c) preparing dispersions of the coating materials and heating the dispersions to about 30 to 90.degree. C.; and (d) aspirating the heated dispersions of the coating materials into the mixing vessel while simultaneously mixing the dispersions with the particulate styrene polymers to form a coating thereon and drying the coated particles. The patent teaches that especially advantageous dispersions contain mixtures of monostearates and distearates of glycerin or the sorbitol ester of palmitic acid as the dispersing agents. Further, mixtures with dispersing agents of metallic soap such as magnesium zinc stearate or calcium stearate bisstearylethylenediamine are also used advantageously.
U.S. Pat. No. 4,020,133 issued Apr. 26, 1977 to T. Altares, Jr. discloses a collagen-aluminum complex which is dispersed in water, serving as a suspension agent for suspending polystyrene beads during impregnation of a blowing agent. The aqueous system is drained from the impregnated expandable beads, which are washed, but which retain adherent deposits of the collagen-aluminum complex. The dry coated beads are said to have minimized propensity toward lumping when pre-expanded.
U.S. Pat. No. 4,018,946 issued Apr. 19, 1977 to M. Klein discloses a method for producing expandable discrete styrene-polymer bit-pieces impregnated with a liquid aliphatic impregnant, wherein a quantity of styrene polymer bit-pieces are mixed in an aqueous solution with a quantity of the aliphatic impregnant sufficient to provide the desired extent of impregnation and a quantity of a compatible water-soluble emulsifying agent sufficient to emulsify the quantity of aliphatic impregnant in the aqueous solution. The patent teaches that suitable emulsifying agents may be any of the neutral soaps or synthetic wetting or dispersing agents such as are used as emulsifying agents in emulsion polymerization of styrene or any monomers used in preparing a styrenepolymer. Among these applicable emulsifying agents are sodium or potassium salts of fatty acids, anionic wetting agents such as sodium dodecyl sulfate, and cationic emulsifying agents such as quaternary ammonium salts.
It is, therefore, a primary object of the present invention to provide methods of making antistatic expandable and expanded vinylaromatic polymer-containing articles with significantly decreased propensity for acquiring static electricity, especially when the articles are expanded to substantially the maximum amount so that they may be used for inexpensive packing material.
It is also an object of the present invention to provide such methods that (1) are not deleterious to components in the articles, (2) provide a durable and uniform treatment, (3) are less costly in terms of materials and labor, (4) are easy to perform, and (5) avoid nucleation during extrusion of the unexpanded articles.
It is yet another object of the present invention to provide methods which provide good antistatic qualities to expanded vinylaromatic polymer-containing articles. One indication of good antistatic quality is when the expanded articles dissipate an induced 5000 volt charge to 0 volts in less than 2 seconds. It is also an object of the present invention to provide articles which are produced by such methods.